Ink jet printing is performed by discharging ink droplets from a print head to a substrate. The droplets are ejected through orifices or nozzles in the print head and are directed to the substrate to form an image thereon. In contrast to many other types of printing, there is no contact between the printer and the substrate in ink jet techniques.
Most of the ink jet printers known in the art may be characterized as either continuous or impulse devices, depending upon the mechanism by which the ink droplets are directed to the substrate. In continuous ink jet systems, an essentially uninterrupted stream of ink is ejected from a nozzle and breaks up into droplets. The droplets bear an electric charge so that they can be deflected by an applied electric field which is modulated according to the particular image to be recorded. The electric field directs the droplets toward either the substrate or an ink re-circulating reservoir. The inks employed in conjunction with continuous ink jet systems typically comprise a colorant such as a dye or pigment, an electrolyte to facilitate droplet deflection, and a liquid vehicle to dissolve or disperse the colorant and the electrolyte. Typically, the vehicle comprises water, an organic solvent, or a mixture thereof. The water is sometimes used to control the viscosity of the ink.
With so-called "impulse" or "drop-on-demand" ink jet printers, image formation is controlled by selectively energizing and de-energizing a piezoelectric transducer rather than by modulating an applied electric field. Ink is stored in the print head or nozzle until it is necessary to form an image on the substrate. The printer is then activated to apply pressure to the ink and discharge a selected number of discrete ink droplets toward the substrate. These ink droplets need not bear an electric charge. Accordingly, impulse ink compositions are free of corrosive substances such as water and electrolytes which continuous stream inks often comprise.
However, impulse ink jet printers present a number of problems which are not encountered in continuous ink jet systems. For example, unlike continuous ink jet printers, impulse printers typically are maintained in a stand-by or temporarily inoperative mode between printing cycles. Thus, the ink is allowed to stand and possibly solidify in the discharge orifices of the print head. Impulse printers normally begin a printing cycle with such material in place. Many of the start-up problems encountered with impulse printers are attributable to ink which has been allowed to stand in the discharge orifices during stand-by periods. Such material is less of a concern in continuous systems because there typically are fewer interruptions in the flow of ink. Even where ink is allowed to stand and solidify, it is more easily purged due to the considerably higher pressures at which continuous ink jet printers operate. Accordingly, impulse-type inks must be specially formulated to minimize start-up problems.
Numerous ink compositions for impulse ink jet printers are known in the art. However, most of these inks are not suitable when the ink is to be used for writing on surfaces such as highly calendared paper which absorb the ink only slowly. As will be appreciated by those of skill in the art, printed images typically cannot become dry to the touch before either the ink solvents have evaporated or the ink has been absorbed by the writing surface. The rapid evaporation of solvents from the ink, however, often leads to clogging of discharge orifices during stand-by periods. Thus, the rate at which writing surfaces absorb impulse ink jet inks provides a critical limitation to the drying of printed matter. Rapid drying is often necessary where objects such as conveyor belts come in contact with freshly printed articles or where the articles are to be stacked shortly after printing.
Therefore, there exists a need for a rapidly drying ink composition for use in impulse-type ink jet printers.